The present invention relates to a digital equalizer apparatus incorporating a digital filter for frequency compensation of an audio signal which has been converted to digital code sample form. In particular, the invention relates to such an apparatus which employs a FIR (finite impulse response) digital filter, and enables mutually independent adjustment of the amplitude/frequency and phase/frequency response characteristics of the filter.
With the equivalent of audio apparatus utilizing digital signals in recent years, digital equalizers have been developed based upon FIR filters. In the following, it will be assumed that a FIR filter is a transversal filter, i.e. a tapped delay line filter. However it should be noted that the present invention is not limited to such an FIR filter, and that other filter configurations can be utilized. The transfer function of such a digital transversal filter, determined by the amplitude/frequency characteristic and phase/frequency characteristic of the filter, is determined by the respective values of a plurality of filter coefficients (sometimes referred to as tap coefficients). Such an FIR filter has been utilized in the prior art for audio digital equalizer. However in the prior art it has not been possible to execute mutually independent control of the phase and amplitude response characteristics of such an audio equalizer by using a single FIR filter, i.e. for thereby independently modifying the amplitude/frequency characteristic and phase/frequency characteristic of a digital audio signal by transferring the signal through the FIR filter.
In addition to such audio equalizer applications, a digital equalizer apparatus based on a FIR filter can be adapted to various other functions, for example suppression of "howl" caused by acoustic feedback between a microphone and a loudspeaker.
FIG. 1 is a system block diagram of an example of a prior art digital equalizer apparatus based on a FIR filter. Numeral 1 denotes an amplitude/frequency characteristic input section, for input of data which represent an arbitrary amplitude/frequency characteristic that will be designated as .vertline.H(.omega.).vertline.. Numeral 5 denotes an inverse Fourier transform section which operates on the input amplitude/frequency characteristic as a transfer function, and derives the inverse Fourier transform of this transfer function. This inverse Fourier transform is an impulse response characteristic corresponding to the transfer function, as described hereinafter, and a set of values of filter coefficients respectively determined by that impulse response characteristic is thereby obtained. Numeral 6 denotes setting means for establishing these values of filter coefficients for a FIR filter 7, to thereby determine the desired amplitude/frequency characteristic for the filter. Numeral 8 denotes a signal input section for converting an input signal to suitable digital signal form to be processed by the FIR filter 7, and 9 denotes a signal output section for converting a digital output signal produced from the FIR filter 7 to a suitable form for transfer to external circuits.
Data representing the desired amplitude/frequency characteristic .vertline.H(.omega.).vertline. are inputted through the amplitude/frequency characteristic data input section 1, as a set of amplitude values corresponding to respective frequencies, referred to in the following as sample frequencies. FIG. 2(A) shows an example of such an amplitude/frequency characteristic, in which these input amplitude values are indicated as black dots, with data being inputted only within a frequency range designated as 0 to .pi.. As shown in FIG. 2(B), the desired amplitude/frequency characteristic in the range 0 to 2 can be derived by "folding over" the portion of the characteristic from 0 to .pi. and thereby obtaining the characteristic in the range .pi. to 2.pi..
The amplitude/frequency characteristic in the range 0 to 2 thus obtained is applied to the inverse Fourier transform section 5, where the inverse Fourier transform is derived. More specifically, the amplitude/frequency characteristic .vertline.H(.omega.).vertline. is treated as if it were the absolute amplitude portion of a transfer function H(.omega.), i.e. EQU H(.omega.)=.vertline.H(.omega.).vertline. (1)
As is well known, the inverse Fourier transform of a transfer function (which is a complex function in the frequency domain) is a time domain function which represents the impulse response of the circuit having that transfer function. Thus, the inverse Fourier transform of the transfer function H(.omega.) is derived by the inverse Fourier transform section 5, to thereby obtain a desired impulse response for the FIR filter 7 corresponding to the input amplitude/frequency characteristic from input section 1. Since the respective values of filter coefficients of a transversal filter are inherently defined by corresponding values of the impulse response of the filter, the appropriate filter coefficient values for the FIR filter 7 are thereby determined. These values are then set in the FIR filter 7 by the setting section 6 (e.g. by control signals applied from section 6), so that the amplitude/frequency characteristic of the FIR filter 7 is thereby made identical to that inputted from input section 1.
The inverse Fourier transform is executed in accordance with the following equation: EQU h(n)=1/N.times..SIGMA.H(.omega.).times.e.sup.j.omega. h (2)
In the above, .omega.=2.times..pi./N.times.k 0.ltoreq.n.ltoreq.(N-1)
The values h(n) obtained from equation (2) are the filter coefficients that are established for the FIR filter 7 by the setting section 6. The FIR filter 7 thereby realizes the specified amplitude/frequency characteristic. However the phase/frequency characteristic of the FIR filter 7 is determined by the transfer function of equation (1) above, and so is fixed as an inherently linear characteristic.
Thus with the prior art example of FIG. 1, although it is possible to realize an arbitrary shape of amplitude/frequency characteristic for the FIR filter 7, the phase/frequency characteristic of the filter is inherently defined by the filter coefficients to be linear. It is thus a disadvantage of such a prior art apparatus that it is not possible to mutually independently establish an arbitrary shape of phase/frequency characteristic and an arbitrary shape of amplitude/frequency characteristic, using a single FIR filter.
In addition to the above, problems also arise even if an equalizer apparatus is implemented which is capable of being adjusted to produce such arbitrary phase and amplitude responses (e.g. by using separate FIR filters for these responses). For example if it is desired that the FIR filter will realize the amplitude/frequency characteristic and phase/frequency characteristic of a specific circuit or system, then it is necessary to first measure that amplitude/frequency characteristic and phase/frequency characteristic of the circuit or system and to then input measured data representing the amplitude/frequency characteristic and the phase/frequency characteristic respectively to respective amplitude and phase input means. Moreover if it is desired to realize, using such a FIR filter apparatus, an amplitude/frequency characteristic and phase/frequency characteristic that have been computed, then there is no simple way of inputting that amplitude/frequency characteristic and phase/frequency characteristic for establishing the desired FIR filter response.